Conventional motor vehicles, such as the modern-day automobile, are built with a powertrain that operates to propel the vehicle and power the onboard vehicle electronics. The powertrain, which is inclusive of and sometimes improperly referred to as a drivetrain, is generally comprised of an engine that delivers driving power to the vehicle's final drive system (e.g., front and/or rear differential, front and/or rear axle, and wheels) through a multi-speed power transmission. Automobiles have traditionally been powered by a reciprocating-piston type internal combustion engine (ICE) because of its ready availability and relatively inexpensive cost, light weight, and overall efficiency. Such engines include 2-stroke or 4-stroke compression-ignited diesel engines and 4-stroke spark-ignited gasoline engines, along with six-stroke and rotary engines.
Hybrid vehicles, on the other hand, utilize alternative power sources to propel the vehicle in order to minimize reliance on the engine for power and thereby increase overall vehicle fuel economy. A hybrid electric vehicle (HEV), for example, incorporates both electric energy and chemical energy, and converts the same into mechanical power to propel the vehicle and power the vehicle systems. The HEV generally employs one or more electric machines (E-machine), such as electric motor/generators, that operate individually or in concert with an internal combustion engine to propel the vehicle. Since hybrid vehicles can derive their power from sources other than the engine, engines in hybrid vehicles can be turned off while the vehicle is propelled by the alternative power source(s).
Series hybrid architectures are generally characterized by an internal combustion engine drivingly coupled to an electric generator. The electric generator, in turn, provides power to one or more electric motors that operate to rotate the final drive members. In effect, there is no driving mechanical connection between the engine and the final drive members in a series hybrid powertrain. The lack of a mechanical link between the engine and wheels allows the engine to be run at a constant and efficient rate, e.g., closer to the theoretical limit of 37%, rather than the normal average of 20%, even as vehicle speed changes. The electric motor/generator may also operate in a motoring mode to provide a starting function to the internal combustion engine. This system may also allow the electric motor(s) to recover energy from slowing the vehicle and storing it in the battery through “regenerative braking.”
Power-split hybrid architectures, by comparison, can be typified by an internal combustion engine and one or more electric motor/generator assemblies, each of which has a driving mechanical coupling to the power transmission. Most power-split hybrid designs combine a large electric generator and a motor into one unit, providing tractive power and replacing both the conventional starter motor and the alternator. One such power-split hybrid powertrain architecture comprises a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving power from the ICE, and an output member for delivering power from the transmission to the driveshaft. First and second motor/generators operate individually or in concert to rotate the transmission's output shaft. These motor/generators are electrically connected to an energy storage device, such as a battery pack, for interchanging electrical power between the storage device and motor/generators. A powertrain system control unit is employed to regulate the electrical power exchange between the energy storage device and motor/generators, as well as the power interchange between the motor/generators.
Electrically variable transmissions (EVT) provide for continuously variable speed ratios by combining features from both series and parallel hybrid powertrain architectures. EVTs are operable with a direct mechanical path between the internal combustion engine and final drive, thus enabling relatively high transmission efficiency and the application of lower cost, less massive motor hardware. EVTs are also operable with engine operation that is mechanically independent from the final drive, in various mechanical/electrical split contributions, thereby enabling high-torque continuously-variable speed ratios, electrically dominated launches, regenerative braking, engine-off idling, and two-mode operation.
An EVT can use differential gearing to achieve continuously variable torque and speed ratios between input and output without sending all power through the variable elements. The EVT can utilize the differential gearing to send a fraction of its transmitted power through the electric motor/generator(s). The remainder of its power is sent through another, parallel path that is mechanical and direct (i.e., “fixed ratio”), or alternatively selectable. One form of differential gearing is the epicyclic planetary gear arrangement. Planetary gearing offers the advantage of compactness and different torque and speed ratios among all members of the planetary gearing subset. Traditionally, hydraulically actuated torque establishing devices, such as clutches and brakes (the term “clutch” used hereinafter to refer to both clutches and brakes), are selectively engageable to activate the aforementioned gear elements for establishing desired forward and reverse speed ratios between the transmission's input and output shafts. The speed ratio is generally defined as the transmission input speed divided by the transmission output speed.
Shifting from one speed ratio to another is generally performed in response to engine throttle and vehicle speed, and normally involves releasing one or more “off-going” clutches associated with the current or attained speed ratio, and applying one or more “on-coming” clutches associated with the desired or commanded speed ratio. Shifts performed in the above manner are termed “clutch-to-clutch” shifts, and require precise timing in order to achieve optimal quality shifting. A shift made from a high speed ratio to a lower speed ratio is referred to commonly as an “upshift,” whereas a shift made from a low speed ratio to a higher speed ratio is referred to commonly as a “downshift.” Shift control includes “power on” shifting and “power off” shifting. Power on shifting refers to a shift operation that takes place during driver “tip-in,” i.e., when the driver depresses the accelerator pedal, while power off shifting refers to a shift operation that takes place during driver “tip-out,” i.e., when the accelerator pedal is partially or fully released.